Well so far The mission builder has worked just fine for me, I was diligent and walked thru making the tutorial mission; "Calais Cleanup" and it was succesfull at showing me the ropes of a basic mission, tho I havent went into the more in depth tweaking of it. I'll get to that as i gain some more exp.
But everything loaded fine and runs fine, I then made another simple one, just an intercept mission consisting of just a simple flight of four Spifires to intercept a flight of four JU-88's heading for a strike at a brittish airfield.
Heh, well except for my lack of attention to timing( I had us arriving a little early at the intercept point,... ok so we were 15min too early) So we just did some loitering till the bombers showed up, but heh, during this loitering I geuss I took us a bit too close to the French coast, and drew AA fire, which in turn must have called up a flight of 109's to jump us just as the bombers showed up.

Well I must say I was pleased, as I didnt even put in any escorts for the JU-88's but wound up having to deal with some anyway, which turned what I thought would be a cakewalk into quite a fight!
And I didnt even try to set it up that way, so yeah, it was cool to have it turn out like that even so.

So to sum up, I like it and im sure that as I gain some exp I'll be able to make some cool & interesting missions, and have fun doing so, I like things that allow this and am pleased with it so far

Did you use my link?  It worked for me.